1. Field of the Invention
This invention relates to pulsed plasma thrusters for spontaneously adjusting the attitude or altitude of a spacecraft. More particularly, this invention relates to a single controller that controls the magnitude and direction of thrust in a thruster set.
2. Description of the Art
Present spacecraft attitude and altitude control systems use a combination of rotating wheels with either thrusters or magnetic torque rods to compensate for changes in wheel speed when the spacecraft attitude is adjusted. The wheels are used to absorb angular momentum or to generate rotation of the spacecraft in response to disturbance torque imparted to the spacecraft. Wheels have been used because a system utilizing only thrusters was considered to be limited by the amount of propellant that may be carried on board.
Thrusters have not been used to their fullest potential because even a very small control thrust generated by typical thrusters is too large and such a thrust caused the spacecraft to be overwhelmed. This required compensation measures to correct the motion of the spacecraft. This problem in spacecraft control is due to the fact that many designs provide thrust in two directions or alternatively, cant the electrode to provide thrust at an angle. These designs are not desirable since thrust is provided in two components and thus, requires firing a second thruster to compensate for any undesired torque or translations produced as a result of the components of the first thrust.
U.S. Pat. No. 4,848,706 (Garg et al.) discloses a spacecraft attitude control system using coupled thrusters. The thrusters provide cross coupling torque so that as one thruster is fired; a second thruster is fired thereby providing a balancing force.
U.S. Pat. No. 5,207,760 (Dailey et al.) discloses a multi-megawatt pulsed inductive thruster. A gas is discharged against an inductor. Each thruster has an associated capacitor to discharge by a trigger generator after a puff of propellant reaches an inductor.
U.S. Pat. No. 5,339,623 (Smith) discloses singly fueled multiple thrusters simultaneously energized by a common power supply. This technique allows a single power supply for multiple thrusters but does not switch from one thruster to a second thruster.
U.S. Pat. No. 5,439,191 (Nichols et al.) discloses a railgun thruster, which provides attitude control via three perpendicular axes. This patent discloses that each thruster has an associated power source.
Thus, there is a need for an improved thruster that provides a single control unit and power source for a plurality of thrusters such that the proper magnitude of thrust is provided to maintain the desired direction of the spacecraft. This type of thruster would eliminate the need for additional thrusters to be fired.